Fuel injection systems for internal combustion engines are well known. Likewise, methods and advantages of pilot fuel injection are known, having been described by Dr. P. H. Schweitzer, "What can be Gained by Pilot Injection" Automotive Industries, Vol. 79 (1938) pp 533-534; G. Monnot et al, U.S. Pat. No. 2,966,079; and P. Eyzat, U.S. Pat. No. 3,439,655. Dr. Schweitzer's article points out some of the advantages obtainable with pilot injection, e.g. elimination of the characteristic diesel knock by reducing the rate of cylinder pressure rise per degree of crankshaft rotation, lower peak cylinder pressure, increased power output, and a reduction in fuel consumption per horsepower hour. Dr. Schweitzer also identifies a problem encountered when attempting to reduce the pilot fuel injection concept to practice; specifically, providing a fuel metering valve which produces a stable pilot charge over the full operating range of the engine.
Since Dr. Schweitzer's contributions, researchers have confirmed the stated advantages of the pilot fuel injection concept and in addition have discovered that the concept can be used to reduce pollutant exhaust emissions, such as oxides of nitrogen, while retaining the stated advantages. However, the problem of economically reducing the concept to practice in a manner that provides all of the obtainable advantages of the concept over the full operating range of the engine has remained unsolved. To obtain all of the possible advantages of the pilot injection concept, the injection system must be capable of accurately metering very small amounts of pilot fuel to the engine cylinder followed by an equally accurately metered main fuel charge; the timing of the pilot and main charges must be variable with respect to the crankshaft timing; the timing to the pilot charge must be variable with respect to the main charge; the amount of the main charge must of course be variable; and in some cases the amount of the pilot charge must be variable to obtain ultimate results.